Re: [TCML] NST Measurements

[bartb <bartb@xxxxxxxxxxxxxxxx>]

Hi Ed,

I was contemplating the best way to describe this situation from a text 
standpoint and without the B-H curve explanations (I had thought about 
going into that, but decided to exempt it from explanation a few emails 
back to prevent further confusion).

Would you agree with the following?

"", the inductance changes because the permeability increases with flux 
density which increases with current. From zero flux density to the flux 
density at which core saturation occurs, the permeability of the core 
increases to the cores saturation point. Flux density can then no longer 
increase core permeability (the core is saturated). As the current is 
increased beyond the saturation point, inductance is reduced drastically 
because the core can no longer provide additional reactance. Thus, the 
only impedance is the resistive portions of the circuit which is what 
causes the reduced inductance beyond the saturation point.

I find it difficult to speak in "correct" terms to get my point across, 
and I guess because I only think about transformer characteristics 
during a TCML discussion.

Would you word the described characteristics differently?

Bart



Ed Phillips wrote:
> The reason of core saturation is correct. But more specifically, it is 
> the permeability that is changing due to saturation and this is what 
> causes the inductance to change. Current through the windings, the emf 
> due to current, and how much it permeates the core causes the 
> permeability of the core to change. Change in inductance occurs 
> because the winding and core permeabilities are magnetically linked 
> when current is flowing.
>
> Does that answer the question?
> Bart"
>
>    A quibble perhaps but it would be more correct to say that the 
> inductance is changing because the permeability increases with the 
> flux density at low input voltage levels which are far from 
> saturation.  At the "correct voltage" the leakage reactance resonates 
> with the external capacitance and THEN the core goes into saturation 
> [if the insulation can take the voltage].  I have a 15 kV, 60 mA NST 
> here which, with an 0.006 ufd HV mica capacitor across the secondary, 
> exhibits that effect at around 30 volts on the primary.  Blew up the 
> HV scale on my Simpson meter in finding it out many years ago.  With 
> the meter on the 5000 volt scale I was increasing the variac and 
> boom.......  Last time I operated a loaded NST without a safety gap.  
> Transformer and capacitor survived OK but blew out the multiplier 
> resistor on the meter.
>
> Ed
>
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